During a plastic molding/compression process, gases are often trapped in the mold thereby adversely affecting the appearance, structure and performance of the article as illustrated in FIG. 2 wherein reference numeral 100 indicates surface imperfections and reference numeral 102 indicates non-fill areas of the article.
For example, as described in U.S. Pat. Nos. 5,516,551 and 5,599,630, sheet molded compound (SMC) parts often have volatiles and/or gases contained therein which, during processing of the part, migrate to the surface of the part. A resin-rich layer adjacent the surface of the part can oftentimes partially block passage of gases from internal voids formed within the SMC articles. However, many processes can disrupt this resin-rich layer to expose these internal voids to the surface of the article.
It is also often necessary to vent the air out of the article-defining cavity of a plastic injection mold, either during a plastic injection step, during a pressurized gas injecting step, or during a step of venting gas from a gas-assisted injection molded part. Prior attempts to vent the cavity are often expensive and/or complex.
U.S. Pat. No. 5,665,281 discloses a method of molding using a venting pin which, in one embodiment, can also serve as an ejector pin.
U.S. Pat. No. 3,555,620 discloses a mold for pressure injected materials in which the ejector pins and their associated board acts as a venting means.
U.S. Pat. No. 5,356,281 discloses a stack mold, including gas ejection of pieces having a reciprocable gas blow-off pin slidably trapped in a pin recess and including an enlarged bottom portion and a narrow upper portion that includes an internal gas passage.
U.S. Pat. No. 5,545,366 discloses an arrangement for molding in which one ejector pin is located at the distal end of a part to ensure adequate mold gas venting during the final fill stage.
U.S. Pat. No. 4,123,218 discloses a blow pin assembly having a narrow gap between a main body and a sleeve to provide a vent for trapped air.
U.S. Pat. No. 3,645,492 discloses an ejector actuator for a plastic injection mold for introducing air or other fluid into an associated cylinder.
U.S. Pat. No. 4,531,703 discloses an air ejector which moves an ejector valve into the mold and injects air between a molded article and the mold to eject the article.
The U.S. patent to Schneider et al., 5,284,429, discloses a mold and gas pin assembly for use in the injection molding of a thermoplastic article. A fluid supply passageway is provided in the mold to allow fluid to be ejected through and communicated to either the thermoplastic flow path or into the article-defining cavity. An end passage of the thermoplastic flow path has a plug disposed therein. The plug includes a porous core which permits the flow of low viscosity fluid such as a gas in either direction therethrough, but prevents the passage of molten thermoplastic therethrough.
U.S. Pat. Nos. 5,169,577, 5,316,605 and 5,217,729 all disclose the use of gas permeable parts for forming plastic articles.